1. Field
Embodiments relate to a fiber-optic surface plasmon resonance sensor and a sensing method using the same.
2. Description of the Related Art
A surface plasmon resonance sensor is a sensor utilizing the phenomenon that the excitation condition of surface plasmons, which are charge density waves of free electrons generated on the surface of a metal film at a metal-dielectric interface, is very sensitive to the change in surrounding environments. Biosensors and environmental sensors based on surface plasmon resonance are extensively studied.
The conventional surface plasmon resonance sensor has a structure including a prism with a high refractive index and a thin layer of a single metal on the basal plane of the prism. When p-polarized light is incident on the prism base with an angle larger than the angle of total internal reflection, surface plasmons are excited by the light incident at a specific angle satisfying the phase matching condition yielding a dip in the reflectance curve. The operation as sensor is achieved by measuring the change of the reflectance curve in response to change in surrounding medium on the surface of the metal film by various means. When monochromatic light is used, the change in the resonance angle at which the surface plasmons are excited may be measured or, with the angle of incidence fixed at the initial resonance angle, the change in the intensity or phase of the reflected light may be measured. When a polychromatic light source is used, the change in resonance wavelength for a particular angle of incidence may be monitored using a spectrometer to detect the change in the surrounding medium.
Such a prism coupler-based surface plasmon resonance sensor has high sensitivity and allows for label-free, real-time reaction analysis. On the other hand, since it requires a high-precision two-axis goniometer and a control system therefor, the cost is high and the system configuration is complicated and bulky. Accordingly, it is not suitable for point-of-care diagnosis or remote sensing.
The fiber-optic surface plasmon resonance sensor proposed in the early 1990s combines advantages from both the fiber-optic sensor appropriate for remote sensing with the high sensitivity of the surface plasmon resonance sensor. Due to the simple system configuration and low cost, it has attracted much attention. For example, U.S. Pat. No. 5,359,681 titled “Fiber optic sensor and methods and apparatus relating thereto” discloses a fiber-optic surface plasmon resonance sensor having a metal layer in contact with an exposed optical fiber core. However, the conventional fiber-optic surface plasmon resonance sensor has several problems.
FIG. 1 is a contour map showing a theoretical calculation result of light reflectance inside the core of a conventional fiber-optic surface plasmon resonance sensor as a function of internal incident angle and wavelength of incident light. The contour map shown in FIG. 1 shows a calculation result for a fiber-optic surface plasmon resonance sensor having a 45-nm thick gold (Au) thin film, which is in contact with a core made of silica, as a surface plasmon excitation layer. The medium surrounding the gold (Au) thin film was assumed to be water.
Since the fiber-optic surface plasmon resonance sensor includes no mechanical moving parts for satisfying the phase matching condition, the internal incident angle in the core is determined by the refractive index of the core and the numerical aperture of the optical fiber. In case of a core made of silica, the minimum acceptance angle of internal incidence, which corresponds to the angle of total internal reflection, for an optical fiber with a numerical aperture of 0.24 is about 80° and that for an optical fiber with a numerical aperture of 0.48 is about 71°.
Considering that the allowable numerical aperture of most of the currently commercially available fiber-optic based spectrometer is about 0.2, an internal incident angle between about 80° and 90° is realistic for the silica-based fiber-optic surface plasmon resonance sensor. Excluding the low incident angle range of impractical high numerical aperture, the surface plasmon resonance wavelength is maintained around 600 nm in spite of the change in the internal incident angle in broad ranges, as shown in FIG. 1. This suggests that the change in resonance wavelength is quite limited even when the multi-mode optical fiber with relatively large numerical aperture is used.
Accordingly, since the operation wavelength is determined by the refractive index of the medium to be analyzed, a light source whose wavelength deviates from the given operation wavelength region cannot be used. Especially, when a single-mode optical fiber is used, the sensor detects the change in signal intensity at a specific position on the reflectance dip curve. In this case, it is very difficult to fine-tune the resonance condition to optimize the signal intensity for the conventional fiber-optic surface plasmon resonance sensor.
In addition, in order to calibrate a signal fluctuation due to external noise factors such as the intensity fluctuation of light source, temperature increase of the measurement system, or the like, it is required to use an additional optical fiber for a reference channel or to form an additional cascade-type reference channel on the same optical fiber. Such requirements cause a burden in process.